scholarly journals A Lower Transition Point for Polymethyl Methacrylate at 30°–32° C

Nature ◽  
1966 ◽  
Vol 211 (5049) ◽  
pp. 627-627 ◽  
Author(s):  
H. F. ATKINSON ◽  
A. A. GRANT
Keyword(s):  
Polymethyl Methacrylate ◽  
Transition Point ◽  
Lower Transition

Nuclear Magnetic and Quadrupole Resonance Studies of Phase Transitions in Crystalline (NH4)2SbF5 and (ND4)2SbF5

1988 ◽  
Vol 43 (3) ◽  
pp. 233-238 ◽  
Author(s):  
Ataru Kobayashi ◽  
Yoshichika Yoshioka ◽  
Nobuo Nakamura ◽  
Hideaki Chihara
Keyword(s):  
Activation Energy ◽  
Phase Transitions ◽  
Transition Point ◽  
Spin Lattice Relaxation ◽  
Ammonium Ion ◽  
Temperature Phase ◽  
Lower Phase ◽  
Resonance Frequencies ◽  
Quadrupole Resonance ◽  
Lower Transition

(NH4)2SbF5 undergoes two successive phase transitions at 169 and 292 K, both of which are of typical lambda-type and without thermal hysteresis. Proton and fluorine spin-lattice relaxation time measurements showed that the isotropic reorientation of the NH4+ ions is highly excited below the lower transition point with an activation energy of 15.5 kJ mol-1 and that the uniaxial reorientation of the square pyramidal SbF52- anion is excited above about 200 K with an activation energy of 42.0 kJ mol-1. There is strong cross relaxation between the proton and the fluorine over the whole temperature range of the measurements. The 121Sb and 123Sb nuclear quadrupole resonance frequencies show an anomalous temperature dependence; each resonance frequency assumes a minimum at around 100 K and increases on heating, reaches a maximum at the lower transition point, decreases drastically in the middle temperature phase and then fades out just below the upper transition point. The anomalous increase in the resonance frequencies stems from the reorganization of the N - H ... F type hydrogen bonds with the progress of the lower phase transition. The lower and the upper transition temperatures shift to 173.0 and 290.1 K, respectively, on deuteration of the ammonium ion.

The Waterproofing Mechanism of Arthropods

1959 ◽  
Vol 36 (2) ◽  
pp. 391-422 ◽  
Author(s):  
J. W. L. BEAMENT
Keyword(s):  
Transition Temperature ◽  
Outer Surface ◽  
Transition Point ◽  
Mechanical Damage ◽  
Absolute Permeability ◽  
Volatile Components ◽  
Terrestrial Arthropods ◽  
Molecular Arrangement ◽  
Lower Transition ◽  
The Individual

1. The relationship between the temperature of the cuticle and its permeability to water has been determined for a number of different terrestrial arthropods 2. In most of these animals the waterproofing wax loses its great impermeability to water very abruptly at a particular transition temperature; that temperature varies appreciably with the individual, but appears to be characteristic of any one species at a particular age within a particular instar. No change in permeability with temperature was found in Tipula sp. taken from soil, in which the cuticle was greatly damaged; above 48° C. the permeability of the cuticle of Schistocerca nymphs changes rapidly with increasing temperature, but this change is not abrupt. 3. In the permeability/temperature curves for larvae of Pieris, Tenebrio, Calliphora and Nematus, pupae of Calliphora and adults of Schistocerca, Ixodes and Ornithodorus only one transition discontinuity is found. In all these animals the wax maintains almost constant permeability up to the point of transition, above which permeability increases very rapidly with temperature. 4. Immediately after moulting, Rhodnius nymphs and pupae of Tenebrio and Pieris also reveal only one transition point; this occurs at a comparatively low temperature. But when cement is secreted over the primary wax layer, a second and higher transition point is found which is independent of the first point. The lower transition temperature seems constant in position during the life of an instar, but the upper point occurs at progressively higher temperatures as the instar ages. The evidence suggests that these animals have two special waterproof layers of wax, the one with high transition temperature occurring on the outer surface of the cement. This outer layer could arise by the migration of wax from the primary layer (with lower transition temperature) through the cement to the outer surface, there to lose volatile components and waterproof the surface. 5. In every animal investigated a permanent increase in permeability is produced by heating a wax layer above its transition point; after such treatment no transition point can be found. 6. From calculations of the absolute permeability of the various wax layers, and from the behaviour of the wax, it is suggested that all epicuticular waxes are laid down with some special molecular arrangement and that in very waterproof insects this may extend throughout the thickness of the wax layers; in less waterproof insects only a portion of the wax layer (as in the cockroach) may be specially organized. 7. Very slight mechanical damage promotes increased transpiration through the cuticle. Stationary adsorptive dusts applied directly to a primary wax layer do not affect permeability; applied to wax above the cement, they promote rates of evaporation in excess of the value exhibited by the same cuticle having a primary wax layer alone. This phenomenon is discussed in relation to the distribution of water within the transpiring cuticle.

Polarized microbeam FT-IR analysis of single fibers

1996 ◽  
Vol 54 ◽  
pp. 206-207
Author(s):  
Liling Cho ◽  
David L. Wetzel
Keyword(s):  
Molecular Orientation ◽  
Transition Point ◽  
Polymer Fibers ◽  
Single Fiber ◽  
Wire Grid ◽  
Polyester Fibers ◽  
Ft Ir ◽  
Ir Analysis ◽  
The Relationship ◽  
Wire Grid Polarizer

Polarized infrared microscopy has been used for forensic purposes to differentiate among polymer fibers. Dichroism can be used to compare and discriminate between different polyester fibers, including those composed of polyethylene terephthalate that are frequently encountered during criminal casework. In the fiber manufacturering process, fibers are drawn to develop molecular orientation and crystallinity. Macromolecular chains are oriented with respect to the long axis of the fiber. It is desirable to determine the relationship between the molecular orientation and stretching properties. This is particularly useful on a single fiber basis. Polarized spectroscopic differences observed from a single fiber are proposed to reveal the extent of molecular orientation within that single fiber. In the work presented, we compared the dichroic ratio between unstretched and stretched polyester fibers, and the transition point between the two forms of the same fiber. These techniques were applied to different polyester fibers. A fiber stretching device was fabricated for use on the instrument (IRμs, Spectra-Tech) stage. Tension was applied with a micrometer screw until a “neck” was produced in the stretched fiber. Spectra were obtained from an area of 24×48 μm. A wire-grid polarizer was used between the source and the sample.

scholarly journals An experimental study of the motion of ice past obstacles by the process of regelation

1976 ◽  
Vol 17 (75) ◽  
pp. 79-98 ◽  
Author(s):  
E. M. Morris
Keyword(s):  
Thermal Conductivity ◽  
Experimental Study ◽  
Cross Sections ◽  
Transition Point ◽  
Water Layer ◽  
Single Object ◽  
Two Systems ◽  
Basal Sliding

AbstractThe results of regelation experiments, in which a single object is pulled through ice, cannot be applied directly to the problem of basal sliding in glaciers because the two systems have different geometries. When the force applied to a single object is small, impurities trapped in the regelation water-layer around the object inhibit the regelation process. At larger forces, above the Drake-Shreve transition point, impurities are shed in a trace behind the object. However, when ice moves over a series of obstacles a trace may exist above and below the transition point. The regelation velocity below the transition point is not reduced by the effect of trapped impurities. In an experiment in which brass cylingerrs of various cross-sections rotate in ice, the ratio between the expected regelation velocity, calculated using the basal-sliding theory of Nye, and the measured regelation velocity is 8±2, both above and below the transition point. The same ratio has been obtained by other workers with wires of similar thermal conductivity above the transition point. Measurements of température differences indicate that supercooling cannot be the main source of the unexpectedly low regelation velocities above the transition point.

scholarly journals Research and Prediction of Wettability of Irregular Square Column Structure on Polymethyl Methacrylate (PMMA) Surface Prepared by Femtosecond Laser

Coatings ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 529
Author(s):  
Bangfu Wang ◽  
Juan Song
Keyword(s):  
Contact Angle ◽  
Femtosecond Laser ◽  
Prediction Model ◽  
Polymethyl Methacrylate ◽  
Groove Width ◽  
Complete Wetting ◽  
Column Structure ◽  
Pmma Surface ◽  
Incomplete Wetting ◽  
Wetting Model

Based on the contact angle prediction model of a traditional square column structure, the prediction models for wettability of a parallelogram square column structure (PSCS) on polymethyl methacrylate (PMMA) surface prepared by femtosecond laser were established. An experiment was conducted to analyze the rationality of the established complete wetting model and incomplete wetting model. It was found that the incomplete wetting prediction model of the square column structure was more in line with the actual situation. For PSCS, the length of both the long and short sides of the boss and the width of the groove exerted an impact on the contact angle prediction results. Under the condition that the length of the long and short sides of the boss remained unchanged and the groove width increased, the contact angle increased under complete wetting and incomplete wetting. In contrast, under the condition that the long side length of the boss and the groove width remained unchanged and the short side length of the boss increased, the contact angle increased under complete wetting but decreased under incomplete wetting. The maximum contact angle reached 135.65°, indicating that PSCS on PMMA surface enhanced the surface hydrophobicity of the material.

scholarly journals Thermodynamic and Kinetic Characteristics of Combustion of Discrete Polymethyl Methacrylate Plates with Different Spacings in Concave Building Facades

Polymers ◽  
2021 ◽  
Vol 13 (1) ◽  
pp. 167
Author(s):  
Weiguang An ◽  
Lujun Peng ◽  
Minglun Cai ◽  
Kaiyang Hu ◽  
Song Li ◽  
...  
Keyword(s):  
Polymethyl Methacrylate ◽  
Flame Spread ◽  
Structure Factor ◽  
Turning Point ◽  
Fire Risk ◽  
Flame Height ◽  
Hazard Evaluation ◽  
Spread Rate ◽  
Building Facades ◽  
Flame Spread Rate

Polymethyl methacrylate plates are widely applied to buildings, producing significant fire hazards. It lacks a theoretical basis for the fire risk assessment of polymethyl methacrylate in concave building facades. Therefore, experimental methods are used to investigate combustion characteristics of discrete polymethyl methacrylate plates in a concave building facade. Influences of fuel coverage and structure factor are investigated, which is scant in previous works. When structure factor is invariable, average flame height increases first and then decreases as fuel coverage increases, and the turning point is between 0.64 and 0.76. In total, three different patterns of pyrolysis front propagation are first observed for different fuel coverages. Flame spread rate first increases and then decreases as fuel coverage rises, and the turning point is also between 0.64 and 0.76. When fuel coverage is invariable, the flame spread rate first increases and then decreases with increasing structure factor, and the turning point is 1.2. A model for predicting the flame spread rate of discrete polymethyl methacrylate is also developed. The predicted values are consistent with experimental results. Fuel spread rate of discrete polymethyl methacrylate rises as the fuel coverage increases. The above results are beneficial for thermal hazard evaluation and fire safety design of polymethyl methacrylate used in buildings.

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